1. Field of the Invention
The present invention is directed to novel methods for diagnosing, prognosing, monitoring the progress of, and treating disorders arising from disregulation of the immune system (e.g., autoimmune disorders, inflammatory diseases, and transplant rejection, and cancer and infectious diseases) related to glucocorticoid-induced TNF receptor (GITR) and the ligand associated with GITR (GITRL) and modulators related thereto. The present invention is further directed to novel therapeutics and therapeutic targets, and to methods of screening and assessing test compounds for the intervention (treatment) and prevention of disorders arising from disregulation of the immune system, as related to GITR and GITRL.
2. Related Background Art
Generally, T lymphocytes are responsible for cell-mediated immunity and play a regulatory role by enhancing or suppressing the responses of other white blood cells. The notion that T lymphocytes play a role in suppression of the immune response is well known (see, e.g., Gershon et al. (1970) Immunology 18:723-35). However, the target antigens for these suppressor cells and the mechanisms controlling their function are still subjects of study.
One population of regulatory T cells that is generated in the thymus is distinguishable from effector T cells by the expression of unique membrane antigens. These regulatory T cells make up a subpopulation of CD4+ T cells (i.e., T cells that express the CD4 antigen) that coexpress the CD25 antigen. CD25 is also known as the interleukin-2 receptor (IL-2R) α-chain. Cotransfer of, or reconstitution with, CD25+ T cells is associated with prevention of both inflammatory lesions and autoimmunity in various animal models (see Shevach (2000) Ann. Rev. Immunol. 18:423-49, and references therein). CD4+CD25+ T cells have also been associated with inhibition of T cell activation in vitro, and adoptive suppression of CD4+CD25− T cells in coculture (Shevach, supra).
More than two decades ago it was demonstrated that some self-reactive T cells escape mechanisms of central tolerance and exist in the periphery under the control of thymic-derived regulatory T cells. In 1995, Sakaguchi and colleagues demonstrated that a small population of CD4+ T cells that naturally express the α-chain of IL-2R (i.e., CD25) are involved in the control of organ-specific autoreactive T cells (Sakaguchi et al. (1995) J. Immunol. 155:1151-64). Specifically, they demonstrated that transfer of CD4+CD25− T cells to immunodeficient hosts led to a spectrum of autoimmune diseases, which could be prevented by cotransfer of CD4+CD25+ T cells (Sakaguchi et al., supra). Subsequent studies have implicated CD4+CD25+ regulatory T cells in the suppression of immune responses to viral, bacterial and protozoal infections (Aseffa et al. (2002) J. Immunol. 169:3232-41; Belkaid et al. (2002) Nature 420:502-07; Hisaeda et al. (2004) Nat. Med. 10:29-30; Kursar et al. (2002) J. Exp. Med. 196:1585-92; Lundgren et al. (2003) Infect. Immun. 71:1755-62; Maloy et al. (2003) J. Exp. Med. 197:111-19). Together, these studies provided evidence that removal of CD4+CD25+ T cells enhanced the immune response. Many attempts have been made to define the activation of, and suppression by, these CD4+CD25+ T cells. These cells represent a unique lineage of thymic-derived cells that potently suppress both in vitro and in vivo effector T cell function.
Several in vitro studies revealed that CD4+CD25+ cells suppress proliferation of CD4+ T cells in response to both mitogens and antigens by turning off transcription of IL-2 (e.g., Thornton and Shevach (1998) J. Exp. Med. 188:287-96; Takahashi et al. (1998) Int. Immunol. 10:1969-80). Cotransfer of CD4+CD25+ T cells in vivo with autoreactive CD4+ T cells is sufficient to suppress both the induction and effector phase of organ-specific autoimmunity (Suri-Payer et al. (1999) Eur. J. Immunol. 29:669-77; Suri-Payer et al. (1998) J. Immunol. 160:1212-18). Other properties of the CD4+CD25+ T cells include hyporesponsiveness to T cell receptor (TCR) stimulation in the absence of exogenous IL-2, immunosuppression via cell-cell interaction, and a requirement for TCR signaling to induce their suppressive phenotype (once they have been activated, however, their suppressive function is independent of antigenic stimulus). It has also been demonstrated that the mere acquisition of CD25 expression, as can be achieved by stimulation of CD4+CD25− T cells, does not induce the suppressive phenotype. These CD4+CD25+ T cells are known to exist in humans (Shevach (2001) J. Exp. Med. 193:F1-F6).
One study demonstrated that altered thymic selection is required for generation of regulatory CD4+CD25+ T cells (Jordan et al. (2001) Nat. Immunol. 2:301-06). In addition, studies with knockout mice demonstrated that molecules involved in IL-2 synthesis and responsiveness are required for generation of these cells; mice genetically deficient for IL2 or IL2Rβ, or B7.1 (CD80) and B7.2 (CD86), or CD28 all have severe reduction in CD4+CD25+ cells, with resulting lymphadenopathy and hyperproliferation in the periphery of some of these mice (Papiernik et al. (1998) Int. Immunol. 10:371-78; Salomon et al. (2000) Immunity 12:431-40; Kurnanogoh et al. (2001) J. Immunol. 166:353-60).
Until recently, the art had failed to determine the mechanisms involved in CD4+CD25+-mediated suppression of the immune system, e.g., the antigen specificity, the molecules involved in acquisition of suppression, and the cell surface molecules or short acting cytokines involved in the effector phase of suppression; the molecular targets of CD25+ T cells in modulating autoimmunity remained largely unknown as well. It has now been demonstrated, by examining differential expression of genes through the use of gene chip analyses on CD4+CD25+ and CD4+CD25− T cells, that several CD25+ differential genes exist (McHugh et al. (2002) Immunity 16:311-23; see also U.S. patent application Ser. No. 10/194,754, incorporated herein by reference in its entirety). These genes, determined to be preferentially expressed on the CD4+CD25+ T cells, can serve as targets for therapeutic intervention and screening methods for autoimmune disorders, inflammatory diseases and transplant rejection, as well as for cancer and infectious diseases.
Significantly, one of the genes determined to be differentially expressed in CD25+ cells is glucocorticoid-induced TNF receptor (GITR) (McHugh et al., supra). GITR, a cell-surface, transmembrane protein receptor, is a member of the tumor necrosis factor receptor (TNFR) superfamily. GITR has been demonstrated to be constitutively present on nonactivated T cells (Gavin et al. (2002) Nat. Immunol. 3:33-41; McHugh et al., supra; Shimizu et al. (2002) Nat. Immunol. 3:135-42). GITR binds to another transmembrane protein referred to as GITR Ligand (GITRL). Agonistic antibodies to GITR have been shown to abrogate the suppressor activity of CD4+CD25+ T cells, demonstrating a functional role for GITR in regulating the activity of these cells (McHugh et al., supra). Another study confirmed that stimulation of GITR with a specific monoclonal antibody abrogated CD4+CD25+ T cell-mediated suppression, thereby inducing autoimmunity (Shimizu et al., supra). These studies have led to the proposal that GITR is a more faithful marker of CD4+CD25+ T cells (Uraushihara et al. (2003) J. Immunol. 171:708-16); however, GITR expression alone does not exclusively distinguish this subset, as upregulation of GITR also occurs following activation of CD4+CD25− T cells (McHugh et al., supra; Shimizu et al., supra).
Because GITR has been shown to be important in the regulation of suppressor activity of CD4+CD25+ T cells on CD4+CD25− T cells, it is desirable to identify and characterize novel molecules that interact with GITR. Such novel molecules that interact with GITR are disclosed herein. Additionally, modulators of these molecules are provided.